136 The Under-Water World 



The elongate Portuguese Oyster and the 

 American Oysters, which differ specific- 

 ally from the British, French and Dutch 

 specimens, do not change sex, remaining 

 either male or female for the whole of 

 their lives. 



The Pearl Oyster has ever been 

 popular, not only for the marketable 

 pearls that about one oyster in twenty 

 contains, but also, in the case of those 

 inhabiting the Australian seas, for the 

 " mother-of-pearl " lining of the shells. 

 The genuine " oriental pearls " are formed 

 of successive layers of nacre, produced 

 by the mantle and wrapped round some 

 foreign substance which has intruded 

 itself. Millions of pounds have been 

 invested annually in order to cope with 

 the demand for real " pearls." Some 

 years ago an enterprising Japanese 

 brought many of the pearl gamblers to 

 earth by realising the obvious, namely, 

 that pearls could be cultured without 



